1. Field of the Invention
This invention relates to austenitic stainless steels which are useful in propeller shafts, pump shafts, motor shafts all for ships and shafts for agitators and which have a high corrosion fatigue strength, loading endurance, corrosion resistance in seawater, and ductility. The present invention also relates to a method for producing such steels.
2. Description of the Prior Art
Known steels used for the propeller shafts, pump shafts and motor shafts for ships are SUS 304, SUS 316, SUS 630, and SUS 329 stainless steels (Japanese Industrial Standard). However, these steels are unsatisfactory in corrosion fatigue strength and are not satisfactory when used in an environment such as in seawater or city water where pitting corrosion is produced. For instance, SUS 304 has a corrosion fatigue strength of about 18 kgf/mm.sup.2, a pitting potential of about 280 mV and an endurance of about 27 kgf/mm.sup.2, which are low in all characteristics. SUS 316 in which 12% of Ni with 2.5% of Mo being contained has a pitting potential of about 420 mV and has thus a good resistance to corrosion in seawater, though Ni content of SUS 316 is greater than that of SUS 304. However, its corrosion fatigue strength of about 20 kgf/mm.sup.2 and endurance of about 28 kgf/mm.sup.2 are not so high. Moreover, SUS 630 in which 4.5% of Ni, 3.5% of Cu and 0.35% of Nb are contained has a good corrosion fatigue strength of about 32 kgf/mm.sup.2 and a good endurance of about 102 kgf/mm.sup.2, but its pitting potential is about 170 mV, so that the resistance to corrosion in seawater is thus poor. SUS 329 JI, which is an austenite-ferrite two-phase stainless steel composed of 25Cr-4Ni-1Mo, has a high pitting potential of about 550 mV and thus, exhibits a good resistance to corrosion in seawater, but has a low corrosion fatigue strength of about 28 kgf/mm.sup.2 and a low endurance of about 48 kgf/mm.sup.2. As will be seen from the above, the conventional stainless steels are not satisfactory with respect to all the characteristics including the corrosion fatigue strength, corrosion resistance in seawater and endurance.